Automatic line testing and switching circuit



H. B. BREHM ETAL 2,680,162 AUTOMATIC LINE TESTING AND SWITCH NG CIRCUIT 6 Sheets-Sheet l ATTORNEY June l, 195.4

Filed Feb. e, 1953 June l, 1954 H. B. BREHM ErAL AUTOMATIC -LINE TESTING AND swITcHING CIRCUIT ATTORNEY June 1, 1954 B, BREHM ETAL 2,680,162

AUTOMATIC LINE TESTING AND SWITCHING CIRCUIT Filed Feb. 6, 1953 6 Sheets-Sheet 3 co/vmoL BVE reMAz.

,A TTORNEY June 1, 1954 H. B. BREHM ETAL AUTOMATIC LINE TESTING AND SWITCHING CIRCUIT Filed Feb. 6', 1953 e sheets-sheet 4 June 1, 1954 H. B. BIREHM ETAL AUTOMATIC LINE TESTING AND swITcHING CIRCUIT Filed Feb. 6, 19753 6 Shee'ts-Sheet 5 H B BREHM J P K//VZER I4 A SMITH /NI/E N 7' ORS BVM( A TTORNEV June l, 1954 H. B. BREHM ErAL 2,680,162

AUTOMATIC LINE TESTING AND swITCHINC CIRCUIT Filed Feb. e, 195s ves sheets-sheet e A. A. SMITH G. WILSON 5V .fd ATTORNEY Patented June 1, 1954 UNITED STATES PATENT OFFICE AUTOMATIC LINE TESTING SWITCHING. CIRCUIT and Ira G. Wilson, New York, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 6, 1953, Serial No. 335,514 9 Claims. (Cl. 179-1753) The invention relates to broad frequency band of another (spare) line which the monitoring test transmission systems, such as multichannel carhas indicated is in tolerable working condition place of the faulty line. In Such Systems employed for transmitting Objects of the invention are to improve autosignal Waves over a long distance it is customary matic line switching arrangements of the aboveto use a large number of repeaters in tandem at describe general type particularly from the attended and others unattended. The failure or 10 in maintenance and operation of the system for a conductor in a line section between repeaters, may thus making available a larger number of lines in result in intolerable degradation of signal trans- 15 each cable for Working lines; and to increase mission over the system. In certain carrier Wave the speed of switching with little increase in systems in which a single line pair or coaxial switching circuit cost or complexity.

It is also known in the prior art, to employ a 45 and each of these line sections including the same repeater switching section and, when the condi- 50 ing repeater station and at the terminal stations. tion of a monitored pilot at the output of the The equipment at each main switching repeater working line indicates a serious faulty condition station includes a plurality of pilot indicators or" that line, to cause its receiving end to be disbridged across the output of each of the parallelconnected from the associated Working line in the connected lines, in the preceding switching secfollowingvrepeater section and the receiving end a5 tion, which are respectively adapted for picking or from the associated line and monitoring the energy in a diiferent one of a number of the pilot signals received over that line. An abnormal change in the power level of any one of the monitored pilot signals indicating a serious faulty condition of the associated line in the same switching section, controls the switching of a spare line into the through signal transmission path in place of the faulty line.

Since only one spare line is provided for several working lines in each switching section, and, in the general case, both ends of a faulty line and the spare therefor are to be switched, separate switches for this purpose are required at the transmitting and receiving ends of each switching section, and the receiving end switch must be delayed after a pilot indicator at the receiving end of the switching section indicates by its condition that a switch to a spare line should be made until information can be sent back to the transmitting end to make a switch thereat. The operation and synchronization of the line switching equipment at the transmitting and receiving ends of each switching section for this purpose is accomplished by the provision of means at each main switching repeater station under control of any one of the pilot indicators associated with the receiving end of a faulty line in the preceding switching section for sending back a switching tone of a distinctive frequency identifying the faulty line, which is different for each line in the switching section, over the signal transmission lines (coaxials) transmitting in the opposite-direction to the main swi ching repeater station at the transmitting end of the switching section to properly control the transmitting switching equipment thereat, means being provided for confining the switching signal to the line section in which a fault has occurred. The proper operation of the line switching equipment at the receiving end of the faulty line is then made under control of a verifier tone sent out over the spare line from the transmitting end when the switching equipment thereat is operated. The

switching equipments are arranged to return the system to the normal condition in which the spare line is disconnected at its transmitting and receiving ends from the working lines of the preceding and following switching sections, re-

spectively, and the normal working line is reconnected at both its ends to the working lines of these switching sections, respectively, when the faulty condition of the normal working line has been corrected.

The switching equipment at the terminal stations of such a signal transmission system diiers from that used at each main switching repeater station only in that the pilot indicators and the receiving switch equipment associated with the lines transmitting in one direction and the transmitting switching equipment and the verifier tone generating equipment associated with the lines transmitting in the opposite direction, used in the latter station, are eliminated.

A feature of the invention is the use of a relatively simple and inexpensive control circuit, which is essentially electronic, in a line switching arrangement of the above-described type for providing relatively fast line testing and switching as compared to that employed with the best of the prior art switching circuits for this purpose.

Another feature is the provision in a line switching arrangement of the above-described type, of means for giving priority as regards switching to a spare line, to one working line over the others in each switching section; and for establishing lesser, different priorities for the other working lines in the same switching section.

The various objects and features of the invention will be better understood from the following detailed description of one embodiment thereof when read in conjunction with the accompanying drawings in which:

Fig. 1 shows diagrammatically a portion of a two-way broad-band carrier communication system equipped with line testing and switching equipment in accordance with the invention;

Fig. 2 shows a simplified schematic of a portion of the carrier communication system of Fig. 1 in which the testing and switching equipments in accordance with the invention associated with one of the corresponding switching sections for the two directions of transmission of the system of Fig. 1 are shown in more detail, which schematic is used in connection with a brief description of the essential principles of the invention;

Figs. 3 to 6, in combination, show in block schematic form the testing and switching equipments in accordance with the invention which would be used at each of the main switching repeater stations in the system of Fig. l;

Fig. 7 shows in more detail a portion of a pilot pick-off circuit (pilot indicator) and one form of switch initiator circuit which may be used in the blocks so labeled in the receiving control equipment at each main switching repeater station as shown in Figs. 3 to 6; and

Figs. 8 and 9 respectively show schematically forms oi transmitting and receiving switching equipment which may be used in the boxes so labeled in each main switching repeater station in accordance with the invention shown in Figs. 3 to 6.

Although the arrangements of the invention are adapted for use with any broad-band signal transmission system employing any type of line, it is particularly adapted for and will be described in connection with a two-way multichannel carrier system employing a type of line structure comprising a tubular conductor within which a central conductor or wire is mounted coaxially by means of insulating supports, commonly referred to as a coaxial or a coaxial pair, and in which a number of these coaxial pairs respectively serving as regular (working) and reserve (spare) lines are contained within a common outer tubular conductor to form what is known as a coaxial cable or pipe The particular coaxial system to be described for which the line testing and switching arrangements of the invention were devised, operates on a four-wire basis, transmission in opposite directions being eiiected on separate ccaxials. This system will provide on one pair of coaxials alternatively, either 1800 two-way telephone circuits; 600 telephone circuits plus a high-grade, twoway, fl-megacycle television circuit; or, by using the entire frequency band, a single two-way rI- megacycle television circuit suitable for wideband color or theater television. This system provides for the transmission of the message and television services at line frequencies between 312 and 8,284 kilocycles over a distance up to 4,000 miles. The repeaters in the system are located at nominal intervals of about 4 miles, and the length of each switching section extending between two main switching repeater stations iS about 300 miles or less. Pilot waves of the dif- 'ferent frequencies 64, 556, 2,064, 3,096, 7,266 and repeater sections and the east-to-west switching sections SSIW and SSZW. Each of the west-to-east switching sections SSE SS-'is and each of the east-to-west switching sections SSiw SSiw includes three paralleled sections of lines, two of which, wZi and w12, are regular or working lines and the third of which sl, is a reserve or spare line for each of the other two lines. The lines wil, w12 and si in each switching section includes a like number of similarly spaced unattended repeaters R and the necessary associated power supplies and regulating equipment (not shown) therefor. Of the pilot waves normally transmitted over the system, those of the frequencies 64, 556, 2,064 and 3,096 kilocycles continuously transmitted over each of the lines wli, w32 and sl for each direction of transmission are used for monitoring these lines in the manner to be described later.

The line testing` and switching equipment in switching repeater station MRI includes control equipment represented by the box I associated with the receiving ends of the lines in the switching section SSiE; control equipment represented by the box 2 associated with the transmitting ends of the lines in switching section SSEE; control equipment represented by the box 3 associated with the receiving ends of the lines in the switching section SSSW; and control equipment represented by the box d associated with the transmitting ends of the lines in the switching section SSdw, as indicated. Similarly, the line testing and switching equipment in accordance with the invention at the main switching repeater station MR2 includes control equipment represented by the box 5 associated with the receiving ends of the lines in the switching section SSQE; control equipment represented by the box 6 associated with the transmitting ends of the lines in the switching section SEEE; control equipment represented by the box i associated with the receiving ends of the lines in the switching section 552W; and control equipment represented by the box ii associated with the transmitting ends of the lines in the switching section SSSW. Similarly, the line testing and switching equipment in accordance with the invention at the main repeater station MRS includes control equipment represented by the box 9 associated with the receiving ends of the lines in the switching section SSSE; control equipment represented by the box it associated with the transmitting ends of the lines in the switching section SSfiE control equipment represented by the box Ii associated with the receiving ends of the lines in the switching section SSI w; and control equipment represented by the box I 2 associated with the transmitting ends of the lines in the switching section SSW.

Fig, 2 shows in simplied schematic form the circuits for correspondingly-located switching repeater sections, say, SSZE and SSSW, for opposite directions of transmission in the system of Fig. 1. The switching sections for the west-toeast direction, as indicated,

in parallel with each other, all of these lines being adapted for transmitting in that direction as shown; and the corresponding switching section for the east-to-west direction may comprise similar lines and their associated transmitting and receiving control equipment (not shown) transmitting in the east-to-west directionl represented in the figure by the dot-dash line LW.

The input of the wor ,ing line wl! is connected through the hybrid coil HI to the output of a working line Li in a preceding repeater switching section, transmitting in the direction from west-to-east carrier waves modulated with message and/or video signals and a number of pilot waves of different frequencies outside the signal frequency range, constituting one carrier channel of the system or" Fig. 1. The input of the working line w12 is connected through the hybrid coil H2 to the outputof a working line L2 in the preceding switching section, transmitting in the direction from west-to-east carrier waves modulated with message and/or Video signals and a number of pilot waves of different frequencies outside the signal frequency range, constituting a second carrier channel on the system of Fig. l. The input of the spare line sl is adapted to be automatically line LI then operating as the working line 1n the preceding switching section, which may be the normal working line cui! or the spare line sl in through the hybrid coil Hi, in parallel with the input of the working line wl I or to the line L2 operating as the second working line in the preceding switching seotion, through the hybrid coil H2, in parallel with the input of the working line w12, under control of switching equipment represented by the switches SI and S2 in the transmitting control equipment TC corresponding to the control equipments 2, d, 6, 8, i0 and I 2 at the main switching repeater stations MRE, MR2 and M123, respectively, in the system of Fig, 1, in the manner to be described later. The hybrid coil HI and the associated line balancing ne"- work NI, and the hybrid coil H2 and the associated line balancing network N2, constitute Wheatstone bridge circuits well known in the art, for enabling transmission between the lines LI and the working line wll and the spare line el, or between the lines L2 and the working line w32 and the spare line sl, depending on the condition of operation of the switches SI and S2 in the transmitting control circuit TC, while providing conjugacy between the inputs of wl! and sl, or between the inputs of w12 and si, respectively. The outputs of the working line wii, the working line wZ2 and the spare line si are adapted to be automatically connected to or disconnected from the lines L3 and L4 in the following switching section, and the line terminating network N3, respectively, by means of the switching equipment represented by the switches S3, Se and S5, respectively, in the receiving switching equipment RS corresponding to the receiving switching equipment in the control equipments I, 3, '5, "i, 9 and i at the main switching repeater stations MRL MR2 and MRS, respectively, in the system of Fig. l.

Separate pilot indicators PIL P12 and P13 are bridged across the outputs of the working lines zoll and wl2 and the spare line sl, respectively. Each of these pilot indicators, the detailed construction of which is shown in Figs. 3 to 6, is adapted to monitor the power in each of a nurnber or the pilot waves of diflerent frequencies transmitted over the associated line, and to be responsive to an abnormal change in the power of any one of these pilot waves such as would be caused by a serious faulty condition of that line in some portion of the transmitted signal frequency range, to control operation of a diderent portion of the control circuits represented by the box CC, as is indicated by the arrow pointing from the output of each pilot indicator towards the box CC. The control circuits CC are included the receiving control equipments i, 3, 5, l, 5 and i! at the main switching repeater stations Miti, and MRT-.5, respectively, in the system of Fig. l. The manner of operation of the control circuits CC, the construction of which is shown detail in Figs. 3 to 6, under control of one or more of the pilot indicators i311, P12 and Pls in response to faulty conditions of the associated lines, to control the switching of the several lines will new be generally described in for various combinations of line failures under appropriate headings.

Normal circuit- Spare line available Under normal operating conditions with no working line faults present, the line sl is available as a spare to either of the working lines wil and w12 on a priority basis. When the line wil is the preferred Working line as regards priori y in switching to the spare line sl in response to a serious line fault, the switches Si and Sil in the transmitting control circuit TC will be normally in the released condition indicated in Fig. 2, with switch Si closed to connect the input of the line sl to the hybrid coil S2 open to eiectively disconnect the input of line sl from the hybrid coil H2. In that case, the pilot wave energy modulated on carrier waves, incoming over the line Ll in the preceding switching section and impressed on the hybrid coil Hi along with communication carrier signals will be divided between the inputs of the lines wll and the spare line sl and will be transmitted over these lines to the inputs of the pilot indicators Fil and P13, respectively, and to the receiving switching equipment RS. For this condition of the circuits, the switches S3, S13 and S5 in the receiving switching equipment RS are in the normal released position shown in Fig. 2 so that the communication and pilot carrier signals received over the line wil or the line wl2 will be transmitted to the lines L3 and Lft, respectively, in the following switching section, and the portions of the communication and pilot carrier signals reaching the output of the spare line sl will be dissipated in the terminating network N3.

Case I .-Fdilure of working line w11 only connection with Fig.

Hi and the switch i associated with these lines,

portion of the control circuits CC in a particular manner. This type of operation will cause a switching tone of a frequency identifying that line, which is diiferent for each working line and within a frequency range, say 50 to 60 liilooycles, below the signal frequency range, to be released 'oy this portion of the circuits CC. This identifying tone will be impressed on the inputs of the transmission lines LW in the corresponding Switching section transmitting in the direction from east-to-west, and will be transmitted over these lines to the input oi the switching section where it will cause operation of the transmitting control equipment TC.

The operation of the transmitting control equipment TC under control of the tone identifying the preferred working line wll, will be such that the switches Si and S2 in the equipment will be maintained in the normal released positions shown, but a veriiier tone, say, of the frequency of 60 lrilocycles, will be released from the control equipment TC and transmitted over the spare line sl to the receiving end of the switching section. This veriiier tone at the receiving end of the switching section will cause operation of the switches S3 and S5 in the receiving switchequipment RS to their alternate contacts, thereby disconnecting the output of the normal working line zoll from the input of the working line L3 in the following switching section and disconnecting the output of the spare line sl from its terminating network N3 and connecting it to the line L3 in the following switching section. Thus the spare line sl will be substituted for the normal working line wll in the through transmission path over the switching section between the line Li at its transmitting end and the line L3 at its receiving end. The switches Si? and S5 in the receiving control equipment RC will be maintained locked operated in their last-mentioned operated positions until the fault on the normal working line wli is removed, after which the switches S3 and S5 will be returned to their normal released positions indicated in `Eig. 2 to reconnect the output of the working line wl! to the lines L3 and to disconnect the output o the spare line sl therefrom and reconnect it to the terminating network N3.

Case II.-Fa1llure of working line w12 only When the working line wll and the spare line sl are in tolerable working condition and a serious fault develops in the working line wl2, the resultant abnormal change in the pilot wave power in the output of the pilot indicator P12 associated with the latter line will cause operation of another portion of the control circuits CC in a different manner. The operated portion of the control circuits CC in this case will cause the release of a switching tone of a frequency different from that identifying the working line wli but within the frequency range 56 to 60 kilocycles, identifying the line w12, which tone will be impressed on the inputs of the lines LW in the corresponding switching section transmitting in the direction from east to west, and will be transmitted over the latter lines to the input of the switching section. At the input of the switching section, the later tone will cause operatlon of the transmitting control equipment TC in the following manner, The switch Si will be opened to disconnect the input of the spare line sl from the hybrid coil Hi and the switch S2 will be simultaneously closed to connect the input of the spare line sl to the hybrid coil H2, thereby disconnecting the input of the line sl from the uput of the line Ll in the preceding switching the input of the switching section at the output of the switching section. The switches Sli and S will be maintained in their last operated positions while the preferred workspare line sl remain goodf line w12 is removed, after which these switches will be re- Case III.-S2Jare Zine failure only When a serious fault develops in the spare line sl, the resultant abnormal change in the pilot n this case, the spare in the terminating net- C'ase IV,-Coe:1zistent failures on working Zines w21 and will-Line wZI fails first or simultaneously with, w22

This case covers the condition where a fault on the working line w12 occurs in the same section containing a fault on the preferred working line wil.

tions in the carrier link until the fault on the preferred working line wZI is removed. The spare line si in each of the following switching sections of the link working line wil working line w12.

is available for the preferred but not for the non-preferred Case V.-Coe:cistent failures on working Zines w11 and wZ2.-wl2 fails first w12 by (l) the transmitted leased by failure of the line zoll and sent back over the east-to-west lines LW, causing the release of the switch S2 in the transmitting control equipment TC to disconnect the input yoi the spare line Case VI.-Coeistent and spare Zine failures on working Zine w11 sL-Lz`nc w11 fails first immedlately and are blocked for the duration of the spare line failure. If the working line wZI later fails, its receiver switch cannot be Automatic restoration When a failed working line becomes or is made good as indicated by normal pilot power on that line (associated pilot indicator not operated), and this power is maintained for about a minute, the switching section will be automatically restored to the normal condition indicated by condition oi the switches Sl to SE in Fig. 2. The delay is inserted in the operation to avoid repeated switches due to an intermittent trouble on the failed line.

Figs. 3 to 6, in combination, with Fig. 3 to the left and Fig. 4 to the right of Fig. 3, Fig. 5 directly below Fig. e and Fig. 6 to the left of Fig. 5 and directly below Fig. 3, show a schematic circuit arrangement or" the line testing and switching arrangements in vaccordance with the invention for each direction of transmission, which would be located at each of the main switching repeater stations MRI, MR?. and MRS in the two-way carrier system o Fig. l. The circuit arrangement as shown in Figs. 3 to 6 is essentially a functional line diagram in which each oi the single lines shown represents a two-wire transmission path and the apparatus elements connected by these paths are illustrated as boxes with suitable labels for indicating their functions. Also, in these Jfigures, an arrow pointing towards a bor; representing a control device from another box indicates that the control device will be operated to perform its vfunction under control o the apparatus represented by the other box. In Figs. 3 to 6, where the control devices represented by the boxes are such as to require a detailed showing of the circuits required therein to perform its adapted for producing such functions is illustrated in detail in one of the other figures, Figs. 7 to 9.

The testing and switching circuits o the invention as shown in Figs. 3 to 6 are essentially electronic in operation, although, for the purposes oi simplication, some oi the transmitting and receiving switching equipment in these figures and in Figs. 8 and 9 as shown employ electromagnetic switching relays. Obviously, switching operations could be performed by the use of other switching arrangements, such as vacuum tubes, trigger gas tube circuits or any of the gating circuits well known in the art, which employ no movable elements in place of such relays, and thus increase the speed of switching.

The lines and associated apparatus at the main switching repeater station of Figs. 3 to 6 corresponding to the similar apparatus shown in Figs. l and 2 bear similar reference characters. All main switching repeater stations are identical but for the purpose of simplifying the description, it will be assumed that the station illustrated in Figs. 3 to 6 is the main switching repeater station MR! of the carrier system of Fig. 1 which connects switching sections SSlw and SSw, and switching sections SSBE and SSllg. Thus, the lines designated wll, w12 and sl transmitting in the direction from west to east, to

the

functions, one form of circuit .L

these coil bridge circuits Hl and H2 (Figs. 4 and 6) are the two working lines and the spare line, respectively, of the repeater switching section SSlE terminating at the station MRI in Fig. l, and the lines designated wZl, w22 and sl to the right of the hybrid coil bridge circuits Hl and H2 are the two working lines and the spare line, respectively, of the following repeater section 832e oi Fig. l, transmitting out of the station MRL Similarly, the lines wil, w12 and sl transmitting in the direction from east to west, to the right oi the hybrid coil bridge circuits Hl and H2 (Fig. 6) are the two working lines and the spare line, respectively, oi the switching section SSSw terminating at the station MRI in Fig. l, and the lines wil, w12 and si transmitting in the direction from east to west, to the left ci the hybrid coil bridge circuits Hl and H2' in Fig. 6 are the two working lines and the spare line, respectively, of the following repeater section SSQFW transmitting out of the station MRl in the east-to-west direction.

The input of the working line wl! of the switching section SSZE is normally connected in wave transmission relation with the output of the working line wil in the switching section SSle, and the input of the working line w12 of section SSZE is normally connected in wave transmission relation with the output of the working line wil of the switching section SSIE, through the hybrid coil bridge circuits Hl and H2, respectively. The input o the spare line SZ of the switching section SSZE is adapted for connection alternatively to the output of the line wil in the preceding switching section SSln or the output oi the line w22 in the preceding switching section SSlE depending on the operating condition ci the trans-- mit switches Sl and S2, respectively, which are under control of the transmitting switching control circuits TSE and TS2, respectively, to be described later. The switches Si and S2 are essentially double-pole, double-throw and may be malre-before-break mercury or reed switches such as are well known in the art or oi any other suitable type. In the case where the line wil in the section SSIE is the preferred line, that is, the one having iirst priority as regards switching to the spare line, the switch Sl is normally closed and the switch S2 normally open, so that the input of the spare line si in the section is normally connected in transmission relation with the output of the wo king line wil in section SSlu through switch Si and hybrid coil bridge circuit Hl, and is normally eiiectively disconnected ircm the hybrid coil bridge circuit H2 and thus from the output or the working line wZ in the section SSln. The connections of the working lines in the repeater sections SSSw and SSlw for the eastto-west direction oi transmission to the right and left of the hybrid. coil bridge circuits Hl and H2' are similar to those described above for the repeater sections SSls and SSE for the west-toeast direction of transmission just described.

The receiving control equipments represented by the boxes l and 3 at the station N35 in Fig. 1 associated with the lines transmitting in the west-to-east and east-to-west directions of transmission, respectively, as shown in Fig. 3 include identical pilot indicator or monitoring circuits Pil, Pl?. and P13 having their inputs respectively bridged across the outputs of the working line wil, the working line w2 and the spare line sl in the switching sections SSIE and SSSw, respectively, through identical coupling circuits including the 1,000;7`5ohm transformer Tl with a the left of the hybrid LOGO-ohm series resistor RI on its high impedance side to reduce impedance irregularities and crosskc. frequency, where the test tone products may create unacceptable spillover on the wider band. The outputs of the filters FI to F4 are paralleled and fed to the input of a common amplifier AI. The amplified pilots in the output of amplier AI are fed in parallel through the relatively broadband filter circuits F5, F5, Fl and F8 respectively tuned to the 64, 556, 2,064 and 3,096 kc. pilot frequencies. The two sets of parallel filters FI to F4 and F5 to F3 in each pilot indicator PIi, P12 and P13 are required to provide adequate discrimination between the individual pilot frequencies, and between the pilot frequencies and the speech channels. The outputs of the respective filters F5, F5, Fl and F8 are fed to the separate rectifiers REI, REZ, RE3 and RE4, respectively, utilized to provide full-wave rectification of each pilot frequency. Each of the rectiers REI to REL! may, for example, use germanium crystals in a conventional circuit. The rectied output of the rectiers RE I to REII are fed from the output of each pilot indicator PII, P12 and P13 to separate switch initiators represented by the boxes SII, S12 and S13, respectively. The switch initiators SII and S13 in combination, for each direction of transmission, comprise control circuits, such as are represented by the box CC in the simplified schematic of Fig. 2.

The function of each of the switch initiators SII and SI2 associated with the output of the pilot indicator PI I and P12, respectively, for the working lines wl! and w12, respectively, of the repeater section SSIE for the west-to-east direction of transmission, and (Fig. 4) with the corresponding working lines of the repeater section SSliw for the east-to-west direction of transmission, is to provide a source of switching tone for identifying the associated working line; for releasing that tone for transmission over the lines transmitting in the opposite direction in the repeater section SSw or SSIE, respectively, in response to an abnormal change (say, i db) in the pilot power output of the associated pilot indicator resulting from a faulty condition of that line; and to provide D.C. voltages for use in controlling the receiving switching equipment of the same repeater section and the transmitting switching equipment of the following repeater section in the manner to be described later.

The function of the switch initiator circuit S13 provide a D.

receiving switching equipment cf that repeater' section, blocking the operation of the switching initiators associated with the working lines of the same repeater section, and for controlling an emergency switch at the transmitting end of the faulty working line under certain conditions in a manner to be described later.

The details of one form of circuit which may be used for the switch initiators SII and SI2 for the abovementioned purposes is illustrated in Fig. '7 associated with the indicator such as The switch initiator SII or S12, as shown in Fig. 7, has four like input circuits I3, I4, I5 and I6 respectively connected across the outputs of the rectiers REI, REZ, RE3 and REi in the associated pilot indicator PII or P12. Two capacitors across the common circuit I 9 which is terminated at one end by the resistors 20 and 2i connected in series, a mid-point between this pair of resistors being connected to ground. One or more oscillators STO have their outputs applied at a mid-point between each pair or capacitors CI and C2 in the input circuits I 3', I 4, I5 and I6, respectively. Each quencies, say of 58 and 56 kc., are used for identifying the particular working line wZI or wl?, respectively, with which the switching indicator and its controlling pilot indicator are associated. Another of these oscillators associated with the switch indicator connected to the preferred certain conditions to be described later. The input of an amplier 22 or 22 is connected across the common circuit I9 through the parallelconnected capacitors 23 and 24, for taking off an alternating tone output from that circuit.

Sources of D.-C. voltage (-24 and are connected as indicated through the series as to apply biasing potentials to each pair of diode rectifiers Ii and I8 in the switch initiator, operating as gating elements, to make their effective impedances such as normally to prevent transmission of the alternating current tone from the switching tone oscillators STO to the encodes common circuit i8. When there is an abnormal change in the pilot output power of one ci the rectiiiers REI to REA in the associated pilot indicator PI! or P12 resulting from a serious fault in the working line wZI or w12 across which that pilot indicator is connected, the resulting voltage applied to the associated input circuit i3, ld, l or i6 of the switch initiator is suiu ricient to overcome the normal biasing potentials applied to the pair of gating diode rectiners il and i8 supplied from that input circuit. This will allow the switching tone from the associated switching tone oscillator or oscillators STO to be applied to the common circuit i9, and this switching tone will be supplied through the capacitors 23 and 2e tc the input of ampliiier 22 or 22', and to the filter Fili also where the switch initiator is Sli associated with the preferred working line wil.

Where the switch initiator operated is Sil associated with the preferred working line wil, alternating tones of both the frequency (say, 58 kc.) identifying that line and of another irequency (say, 50 kc.) obtained from a second oscillator STO connected to the input of the switch initiator Sli will appear in the common circuit i9 of that initiator. Tone of the second frequency (50 kc.) will be selected by the filter Flo and supplied to the input of the emergenoy switching signal buier EB. Also, a portion oi the D.-C. voltage output appearing in the common circuit i9 o Sli will be applied to the emergency signal buffer EB. The tone oi' the frequency (58 kc.) identifying the failed line wZi will he selected and amplified by the aniplier 22 which is tuned to that frequency. Where the switch initiator operated is ST2 associated with the non-preferred working line wifi, a switching tone of the frequency identii'ying that line (say, 56 kc.) only appears in the output circuit le oi that initiator and will he selected by the amplier 22'; corresponding to the amplifier 22 in the output oi switch initiator SII, which is tuned to select that frequency.

Diferent energy portions o the amplified tone (of the frequencies 5S kc. or 55 kc., respectively) appearing in the output oi the amplilier 22 or 22', are transmitted, as shown in Fig. 3 and Fig. 6, from the output of each switch initiator Sli and S12, respectively, over the corninon lead 2 and through parallel circuits respectively including a different one of the resistance loss pads 28 to Sil and a different one of the filters 3l to 33 each having a frequency pass range of 50 to 60 kc. and thus adapted to pass any one of the transmitted tones. The tone of 58 or 5S kc. in the outputs oic the lters Si and 32 pass through the hybrid coil bridges l-ll and H2', respectively, to the inputs of each orthe coaxial lines wli, and mi2, respectively, in the corresponding switching section (SSw) transmitting in the opposite direction, and the tone o 58 or 56 kc. in the output of filter 33 will 'oe passed directly to the input of the spare line oi section SSIIW transmitting in the same direction. The switching tone of 58 or 56 kc. will he transmitted over the westwardly directed coaxial lines of switching section SSliw to the input of that section and thence to the transmitting switch control equipment (TC) for the corresponding eastwardly directed switching section SSiE, as previously described in connection with Fig. 2 and described in more detail in connection with Fig. 3 in another part of the application.

The padding and filtering in the switching tone paths is employed to provide isolation and separation oi the various switching tones, and to maintain necessary crosstall; separation 'between coaxials. The pad losses are such that the tones are fed to the coaxials at the required amplitude. The filters 3i to 33 are designed to pass the frequencies in the to 60 irc. range with about 50 decibels discrimination.

A D.C. voltage is also produced in the coinrnon circuit l@ of the switch initiator Sli or Sli@ 'hen that initiator is operated in response to an abnormal change in the pilot output power of' the associated pilot indicator caused by a serious fault in the working line wil or w12. Portions oi' this voltage are taken ci from the circuit is at any suitable point, as shown in Figs. 3 and a, by the leads 34 and 35 in the case of switch initiator Sli, or leads 3s and 3l' in the case of switch initiator Sli, and are transmitted over these leads to the receiving switching control circuit RS and to the transmitting switching control circuit TSE, and to the receiving switching control circuit RS2 and the transmitting control circuit TS2, respectively, to control these circuits in the manner to be described later.

The switch initiator Slt associated with the output of the spare line SZ of each repeater section (SSla and SSSW) through the pilot indicator PTS is identical with the switch initiators Sli and Slt associated with the working lines oi these sections, the circuit or which is shown in '1, except that in the case 01' the switch initiatcr ST3, the switching tone oscillator STO and the switching tone output circuits employed in the switch initiators Sli and ST2 are eliminated, A ifi-C. voltage appearing in the common circuit i9 of the switch initiator S13 when the rectified pilot applied to one of the inputs of that switcl'i initiator from one of the rectiers RES to in the associated pilot indicator P13 is abnormal due to a serious fault in the associated spare line sl, is taken off from any suitable points in that circuit. This voltage is applied, as shown in Fig. 3, over the leads 3S and 39 to the receiving switching controls RSi and. and to the emergency switching signal huiier EB, for the purposes which will ce described later in connection with these switching circuits.

transmitting control equipment represented by the boxes 2, ai, 6, 8, l0 or i2 at each of the main switching repeater stations MRL MR2 and MRS in the system of Fig. l, also includes verifier tone equipment for supplying a verifier signal at the transmitting end of each section under control of received switching tones, which is transmitted over the spare line of the section to the receiving end thereof to indicate at that end that a switching operation has been carried out at the transmitting end of a faulty section.

This verifier tone equipment, as shown in Figs. l and 6, respectively, includes a source VO, such as a continuously running oscillator, or any suitable type adapted for generating a tone of the frequency, say lrc., below the signal frequency range, used as the verifier tone. The output of the verifier tone source VO is connected in parallel to the input of the verifier buffers VBS and V332 which may be amplifiers normally blocked so as to prevent transmission therethrough of the verier tone (6G kc.) from the output of VO. In this equipment for the west-to-east direction of transmission (see Fig. 4), the output of the verifier buiier VBI is connected through a lter FH having a pass range of 50-60 kc., hybrid coil 17 HI and transmittingswitch SI to the input of the spare line sl for the following repeater section, and the output of verier buier VB2 is connected through the lter FI 8 having a pass range of 50-60 kc., hybrid coil H2 and transmitting switch S2 to the input of the same spare line. The transmitting switch control it is operated to close or maintain closed the transmitting switch Si, is also operative through the delay circuit DC! with a delay of about 50 milliseconds to unblock the verier buffer VBi so as to allow transmission of the 60-cycle tone from oscillator VO through the lter Fl'l, hybrid coil H I and switch SI to the spare line. Similarly, when the transmitting control circuit TS2 is operative to close the transmitting switch S2, it is also operative through the delay circuit DCZ with associated series input for the pilot indicator PII, P12 and P13, respectively, at that station. The lters F9, F I8 the switching tone receiving cirdifferent band widths.

spare line sl passes the frequency band of 50-to-60 kc. This assures the reception of verifier tone from the The switching output of the iilters F9, Fl and Fl I switching repeater station. receiver equipments shown in Fig. 5 controlled from the lines transmitting in the east-to-west direction, as shown by the use of the same reference characters for The receiving switching equipment included equipment represented by the for the West-tc-east direction of transmission at the main switching repeater stations MRi, MR2 and MR3 in the system of Fig. 1, comprises, as shown in Fig. 3, the two receiving switch controls RSI and RSZ, prefor the east-to-west direction of transmission, is shown in Fig. 5. One form which this receiving switchl equipment may have is shown in Fig. 9.

In Fig. 9, the receiving switch control circuit which may comprise an amplifier-rectifier such as is well known in the art, respectively supplied with D.C. control energy from operated switch initiator S13 over lead 38, from switch initiator SI2 over lead 316, from switch initiator S13 over lead 38 and from the verifier tone receiving equipment over lead 40, as indicated. The D.C. energies received from the operated switch initiof RSZ and thus of S4.

ceived from the operated will prevent operation of control circuits RSI The D.C. energy reswitch initiator S13 the receiving switch and RSZ and thus of the lowing switching sectionv through the normally closed contacts 43; and with the terminating resistance All in the working line. wll of the preceding switching section short circuited through theV normally closed switching contacts G of the relay S3. Similarly, in the unoperated condition of the receiving switching control (amplier-rectier) RSZ with no operating voltage being applied to its input. from the switch initiator SI2 and verifier tone receiving equipment in combination, the relay Sil connected to the output of RS2 is unoperated with its switching contacts in the positions indicated in Fig. 9 so that the output of the working line w12 in the preceding switching section is connected to the input of the working line w12 in the following switching section through the normally closed switching contacts 46' of relay S4; and the terminating resistance 4l in the output of working line w12 in the preceding switching section short circuited through the normally closed switching contacts 48 of the relay S6.

The output of the spare line sl of the preceding switching section is normally terminated' by a suitable network, '59, which may he. a resistor as shown, through the normally closed contacts 6c and El of the relay S3 and by the resistor 62 through the normally closed contacts E3 and S4 of relay S4.

When a D.C. voltage is produced in the switch initiator SII by an abnormal change in the pilot output power of the associated pilot indicator PII in response to a serious fault in the normal working line wZI of the preceding switching sece tion, this voltage will be applied to the input of ampliiier-rectiiier RSl. The resulting current in the output of the amplifier-rechner RSI will not be suicient in itself to cause operation of the relay S3 in its output. The operation of that relay will be delayed for the time interval required for the switching tone of the frequency 56`kc. to pass from the initiator SII to the input of the preceding switch section and the verier tone of 60 kc. to return over the spare line sl, to be selected at the output of that line and rectified, and the resulting rectified veriner voltage to be applied over lead c to the input of RSI to supplement the voltage. applied thereto from SII. The relay SB will then operate in response to the increased current in the output of RSI to break its normally closed contacts and close its normally open contacts. The opening of contacts 43 and Ml will cause the connection of the output of line wli in the preceding switching section to the input of the line wll in the succeeding switching section to be broken, and the short circuiting connection around the terminating resistor 45 in the line wil of the preceding switching section to be opened thereby suitably terminating that line. The opening of the normally closed switching contacts 68 and 5I or relay S3 will break the connection of the output of the spare line sl of the preceding switching section to the terminating resistor 59 and the simultaneous closing of the contacts Eil and 66 of the operated relay S3 will connect the output of the spare line si oi the preceding switching section to the working line will of the following switching section wil.

Similarly, the D.C. output voltage produced in the switch initiator SI2 by abnormal change in the pilot output power of the associated pilot indicator P12 in response to a serious fault in the working line w12 of the preceding switching section will be applied to the input of the. am

plier-rectier RSZ. The resulting current in the output of that amplifier-rectifier will not be suflicient to cause operation of the relay S4. The increased current in the output oi the receiving amplifier-detector RSZ when the input thereto is supplemented by the rectified SO-cycle verifier tone energy on lead 4t received over the lines transmitting in the opposite direction from the input of the switching section when the transmitting switches thereat are operated, will result in the operation of relay Sc to open its normally closed contacts and close its normally open contacts. In a manner similar to that described above for operation of relay S3, the opening of its normally closed contacts of relay SII will result in the disconnection of the output of the line w12 in the preceding switching section from the working line w12 in the following switching section and the insertion of the resistance termination 41 in the output oi the line w12 of the preceding switching section; and the disconnection of the terminating resistance network 62 from the output of the spare line sl in the preceding switching section. The simultaneous closing of the normally open switching contacts 5l and Si', of the operated relay S4 will cause the connection of the output of the spare line sl of the preceding switching section to the working line w12 of the following switching section.

The transmitting switching equipment included within the control equipment represented by the boxes 2, I5 and l@ for the west-to-east direction of transmission and the boxes II, 8. and l2 for the east-to-west direction of transmission, respectively, at the main switching repeater stations MRl, MR2 and MRS in the system of Fig. 1, comprises, as shown in 4, two transmitting switch control circuits TSI and TS2, the operations of which are respectively controlled over the leads lill and 55, respectively, by switching tones received over the coaxial lines in the corresponding switching repeater section for the opposite (east-to-west) direction from the preceding main switching repeater station when the switch initiators SII or SI2, respectively, thereat associated with those lines are operated at that station; and the transmitting switches SI and S2 respectively controlled over the leads 5G and 51 from the outputs of the transmitting switching control circuits TSI and TS2, respectively. One orm which this transmitting switching equipment may have is shown in Fig. 8.

In Fig. 8 the transmitting switch controls TSI and TS2 are represented by boxes correspondingly labeled, each of which may comprise an amplifier-rectiiier such as is well known in the art. The transmitting switch SI comprises an electromagnetic switching relay designated by the same reference character, having its operating winding connected across the output of the amplier-rectier TSI; and the transmitting switch S2 by an electromagnetic relay designated by the same reference character, having its operating winding connected across the output oi the amplifier-rechner TS2. In the unoperated condition of the transmitting switch control (amplier-rectier) circuits TSI and TS2 with no switching tone applied to their in puts, the relays Sl and S2 are unoperated and their switching contacts are in the conditions indicated in Fig. 8. In that condition the input of the spare line sl in the following switching section is connected to the working line wil of the preceding switching section through the normal-.-

v the following g rectifier) TSI. .quency 50 kc. will be applied to the input of the the output of -10 and II and 4) through blocked verifier buffer VBI associated with the lead 54 and 2l Vly made contacts 'I0 and 'II of relay SI and hy- 'brid coil HI. The input of the spare line sl of switching section is normally disconnected from the output of the working line w12 in the preceding switching section due to the normally open condition of the contacts 'I2 and 'I3 of relay S2.

In the case of a serious fault on the working line wl I in a switching section for the west-toeast direction of transmission following the main switching repeater station illustrated in Figs. 3 to 6, the operation of the switch initiator SII at the main switching repeater station at the loutput end of that section, will cause switching tones of the frequencies 58 and mitted out over the coaxial lines transmitting in the opposite direction to the input of that section in the manner previously described, At the rinput of this section the rectified tone of 58 kc.

will be applied over the lead 54 to the input of the transmitting switching control (amplifier- The rectified tone of the freemergency control circuit EC (Fig. 4) which is normally blocked and will serve to unblock that Acontrol to allow transmission therethrough of that rectied tone. A portion of the rectified 50 kc. tone in the output of EC will be applied over the lead 85 to the input of the transmitting control (amplier-rectifier) TSI in such manner as 4to apply a blocking bias to that control which is ysuiicient to prevent its operation by the rectified 58 kc. tone applied thereto over the lead 54. Therefore, the switching relay SI connected to switching contacts will be in the normal condition shown in Fig. 8 in which the input of the spare line sl is connected through made contacts hybrid coil HI to the output of the line wll in the preceding switching section. This -spare line sl, therefore, is in condition to receive pilot from the latter working line.

Another portion of the rectied 50-kc. tone in the output of emergency control EC is applied over the lead 86 to the receiving switching control RSI (Fig. 3) and serves to block that control.

Although complete operation of the transmitting switching control TS! in response to the 4gating bias applied over the lead 54 is prevented Vby the blocking bias applied over the lead 85, the control TSI so controlled is conditioned vto apply a D.C. voltage over the lead 8| the delay circuit DCI,

(see Fig. the normally output of the verifier oscillator VO for the westwhich, in the described, renders that bufof the spare line sl for the section in the manner deswitching section will cause the switching of the output ends of the spare line sl and the line wl i lead B5, to closed contacts 81 so'as to break the connection of the switching initiator SII to the transmitting switching control TSI jover the lead 35.

50 kc. to be trans- TSi will be unoperated and its application of the gating pulse over f i trol TS2 Areference characters ying lines wil and wl2 in the corresponding re- 22 The operation of the transmitting switch conand its associated switching relays S2 and SR2 will be described later in connection with a typical switching operation.

The purpose of the emergency switching signal buffers EB (Figs. 3 and 5) associated with the switching initiators Sli and S13 in the repeater section for each direction of transmission is to enable transmission of a :5e-kc. switching tone to the lines transmitting in the opposite direction in response to co-existent failures of the spare and preferred working line wl I. The emergency switching signal buffer EB, which may be a normally blocked amplier or a gating circuit similar to that employed in the switching initiator circuit SII as shown in Fig. 7, is supplied with a 50-kc. tone from the output of the switch initiator SII through the lter FIS when that initiator is operated in response to a faulty condition of the associated working line wli in a repeater section. The buifer EB is unblocked only when a D.-C. output from the switch initiator STI is applied thereto over the lead 88, and simultaneously therewith a D.-C. voltage is applied thereto over the lead 39 from the output of the switch initiator ST3, due to simultaneous operation of both initiators in response to (zo-existent failures of the preferred working line wll and the spare line sl for that switching section. The buffer EB includes means for delaying its being rendered operative for a period of 15 milliseconds after the double failure occurs. When the buifer EB is operated it allows transmission of the applied 50-kc. tone therethrough to the lead 2l' over which it will be transmitted to the coaxial lines in the corresponding repeater section for the opposite direction of transmission, over which it will be transmitted to cause an emergency switch at the transmitting end of the faulty section, which will tie the spare line of that section to the working line w12 thereof (which becomes the emergency preferred linel.

The blocking lters 89 and 90 each having a pass range of 50-60 kc. are inserted inthe working lines wlI and w12 for the west-to-east direction of transmission between the points of connection of the receiving switches S3 and S4 thereto and the input of the hybrid coils H I and H2, respectively. Similar lters having the same are connected in the workpeater section for the opposite direction of transmission betwecni the points of connection and the receiving switches S3 and S4 thereto, and the inputs of the hybrid coils HI' and H2', respectively. The purpose of these line lters is to conne the switching tones to the faulty line in each case while allowing transmission of the carrier'communication and pilot signals.

The other interlocking controls (not numbered) between the apparatus units in the vtesting and switching circuits in accordance with the invention illustrated in Figs. 3 to 6 are provided for performing the functions referred to above for different combinations of fault conditions in' the lines of each repeater section described above under separate side headings.

Typical switching operation a3 sociated with the switching section S521; for the west-to-east direction of transmission in the system of Fig. l. It is assumed that no other faults exist on the other lines in that section. A change in power exceeding 1-5 db at one of the four pilot frequencies 64, 556, 2,664 and 3,696 lic. monitored `at the receiving end or that line w12 initiates the release of a 56-kc. switching tone through the operation of the switch initiator SI2 associated with the pilot indicator PE2 connected to that line. This tone, identiiying the faulty line. is transmitted through amplifier 22 and over the leads 21 through the pads 28, 29 and 34.3, the 50-60 kc. band-pass lters Bti, 32 and 33 and hybrid coils Hl and H2 to all the coaxial lines in the corresponding repeater section SSSW transmitting in the east-to-west direction. The tone passes along these coaxials to the output of that switching section where it is picked oi by the 50-60 kc. band-pass iilters, F3, Fill and Fil bridged across these lines at their receiving ends. The received 5S-lrc. tone after ampliiication Ain amplifier A2 then passes through its individual 2-kc. band-width lter Fifi and rectiiier REt which directs it through lead 55 to the transmitting switch control TS2 associated with the transmitting switch S2 for the west-to-east direction of transmission. When switch relay S2 is operated in response to operation of TS2 by rectified switching tone of the frequency 56 ircA received from the coaxial lines transmitting in `the east-to-west direction in the manner described, the normally closed switching contacts 9| and 92 are opened to disconnect the balancing resistance S3 from hybrid coil H2, and switching contacts 12 and I3 are closed to connect the input of the spare line sZ to the working line w12 of the preceding switching section through these contacts and hybrid coil H2.

The transmitting switch control circuit TS2 when its operates to close the transmitting switch S2 as described above is also operative through delay circuit D02 with a delay of about 50 milliseconds to unblock the verifier buffer VBZ so as to allow transmission therethrough of the verification tone of 60 kc. from oscillator VO through the filter F18 and through the closed switch S2 to the input of the spare line sl in the west-to-east direction over which it will travel to the receiving switch control circuit RSZ at the receiving end of that section. The verifier tone at this point supplies the necessary addition to gating voltage received from the operated switch initiator ST2 to closethe receiver switch S4 and tie the output or" the spare line sl to the working line w12 in the following repeater section, thus completely by-passing the fault.

The switching in sections following the fault is prevented by blocking the transmitter switch S4 in those sections concurrently with the release of switching tone in the preceding sections. The blocking persists long enough for the switching operation to take place, i. e., 5 to l0 milliseconds, after which, the fault in the faulty working line w22 having been closed, that line is again available for switching in these following sections.

The testing and switching equipment at the terminals of a system such as shown in Fig. l would be identical with that used at the main switching repeater station as illustrated in Figs. 3 to 6, except for the elimination of the receiving control equipment including the pilot indicators and switch initiators, in connection with the lines transmitting vfrom west to east, and the elimina- 24 tion of the transmitting control equipment, including the verifier tone generating equipment and the transmitting switch equipment, in connection with the lines transmitting from east to west.

It was determined from calculation of the operating times of the apparatus units including relays, used in the line testing and switching circuits of the invention as described above and the transmission times of the associated transmission lines and circuits, that if fast relays (reed type) were used for performing the nal switching operations and the length of a switching section was approximately 300 miles, the time required for completing the switching of the working and spare lines at both the transmitting and receiving ends of a repeater section in response to a failure occurring in one of the working lines in that section would be about 5 to l0 milliseconds. This compares to a time of about 200 to 250 milliseconds required for switching the lines at one end only of a repeater section in the best of the line testing and switching systems of the prior art brieiiy described above.

The values which have been given above for the frequencies of the pilot waves, switching and verier tones are by way of example only, and it is to be understood that other suitable values for these frequencies and thus for the frequency pass and blocking ranges of the various filters may be employed in the circuits of the .invention. For example, for the particular range of line frequencies for the signals used in the commercial carrier system referred to above, it might be desirable to employ a higher carrier frequency just below the signal frequency range, say, 308 kc., 'for the lowest transmitted pilot frequency, in which case a higher range of frequencies could be employed for the switching tones used, for example, 208 to 296 kc., with a corresponding change in the pass and blocking ranges of the various iilters used. Also, although the testing and line switching circuits of the invention for simplifying the description have been described as applied to a carrier system employing only three coaxial lines in each repeater section one of which is used as a spare, they would apply equally well to a system employing one line as a spare for four to eight lines. Also, although in the system described the verifier tone used for controlling the switching of the lines at the receiving end of each repeater section is generated at the transmitting end of each switching section, it is apparent that a portion of the switching tone transmitted from the receiving to the transmitting end of a switching section in response to a failure or any one of the working lines therein, to control the switching of the lines at the transmitting end of the switching section could be transmitted back over the spare line to control the switching of the lines at the receiving end as well. Other changes in the testing and switching circuits in the testing and switching circuits illustrated and described which are within the spirit and scope o the invention will occur to persons skilled in the art.

What is claimed is:

l. A two-way communication signaling system comprising, for each direction of transmission. a plurality of repeater sections in tandem between terminals of the system, each of said repeater sections comprising a plurality of like transmission lines extending longitudinally over the section in substantially parallel relation with each other two or more of the lines in each re- 25 peater section being regular working `lines each of which is normally connected at its ends to the reserve or spare line for each of the other lines in that section, means for transmitting pilot waves of frequencies outside the signal fre quency range over each of the lines in each repeater section and other means automatically responsive to an abnormal change pilot wave output of that spare line is within normal limits.

2. The system of claim l, in which said pilotresponsive means operates in response to the ab- J normal power output of a faulty line first to switch the faulty and spare lines at their inputs and then to switch them 'at their outputs.

to the output end of the repeater section to thereafter control the switches of the faulty and,

spare lines at their receiving ends.

wave energy therefrom, spare line being normally disconnected from the working lines in the following repeater section, so that a fault in the preferred working line, coexistent with a fault in one of the other working lines, whether it occurs prior to, at the same time or later than that in another working line of the section, will result in the spare line lbeing switched so as to replace the preferred line in the through signal transmission path and the other faulty line= to be disconnected from the spare line.

5. In a two-way broad-frequency band carrier transmission system including terminal stationseach direction of transmission extending between stations, said system being divided into a corresponding number of tandem repeater sections for each direction of transmission, each signals in different frequency ranges, respectively, representing different messages or programs, between corresponding working line portions of the preceding and following repeater section, and an other line portion in each repeater section serving as a reserve or spare line, normally out of service, for each of the working lines of the same section, a switching station located at the junction between each two successive correspondingly located repeater sections for each direction of transmission, means to transmit a plurality of pilot waves of different frequencies outside the signal frequency range over all the line portions in each repeater section for both directions of transmission, separate indicator means at each switching station for respectively picking off and monitoring repeater section, essentially electronic means controlled by the inpilot power output of an associated working line in said preceding repeater section such as would be caused by a serious faulty condition in that line, but only if the pilot power output of the indicator associated with the tion is within normal limits, to cause a switching mission l1nes transmitting in the opposite direction in the correspondingly-located repeater section to said preceding switching station, switching means at said preceding switching station, responsive to the received switching tone to disconnect the input of the faulty line from the working line in the preceding repeater section, normally connected thereto, and simultaneously 27 be switched from said Working line of said first switching priority to said spare line whether or not the faulty condition in said working line of rst priority is initiated before, simultaneously with or after the initiation of the faults in the other working lines o less priority.

8. The system of claim 5, in which means are provided at each switching station operatively responsive to an abnormal change in the pilot output power of the spare line in the preceding repeater section for either direction of transmission such as would be caused by a serious fault in that line to cause the receiving switching equipment controlled by the indicator means associated with each of the Working lines in that repeater section to be released and blocked for the duration of the fault in that spare line so as to prevent switching of through transmis- 28 sion from any of the working lines to the spare line oi that section during that interval.

9. The system of .claim 5, in which normallyblocked sources of switching tone of the distinctive frequencies identifying each of the working lines of the preceding repeater section for each direction of transmission are located at each switching station, all of said distinctive frequencies being in a frequency band below the signal frequency range, the source of distinctive frequency tone identifying a working line being enabled to allow transmission of that tone to the input of that repeater section in response to an abnormal pilot output of the indicator associated with that line.

No references cited. 

